The invention relates to a process for phosphating composite metals containing steel and zinc surfaces and having cavities, using phosphating solutions based on zinc phosphate, by the dipping process.
Steel sheets having an improved surface, in particular galvanized steel sheets, composite metals as they are called, are becoming increasingly important in industrial mass production. Composite metals of this type are obtained by joining steel sheets and galvanized steel sheets and also by partially galvanizing steel sheets. Before painting, especially application of paint by electrodeposition, phosphating with solutions based on zinc phosphate in order to improve corrosion resistance and adhesion of paint is necessary, just as it is for components containing only steel or zinc surfaces.
The special feature in the phosphating of composite metals containing zinc and steel surfaces is the fact that, when contact takes place with the phosphating solution, particularly in dip treatment, an electrochemical cell is formed, with the zinc surface as the anode and the steel surface as the cathode. As a result, in the immediate neighborhood of the zinc surface, the cathodic current density on the steel surface is so high that the phosphating process is adversely affected (W. Rausch, "Chemische Oberflachenbehandklung von verzinktem und zinkstaublackiertem Stahl fur die kathodische Elektrotauchlackierung" ("The chemical surface treatment of galvanized steel and steel painted with zinc dust for the application of paint by cathodic electrodeposition") Industrie Lackierbetrieb 49 (1981), pages 413 et seq.).
Thus, in phosphating by dipping, the formation of a phosphate layer is interfered with to such an extent that, depending on the phosphating process, a strip about 5 to 10 mm wide, carrying a very translucent and only incompletely formed phosphate layer, is formed on the steel side.
Spraying and spraying/dipping processes present fewer difficulties because the formation of an electrochemical cell is incomplete. The strip mentioned above is generally not formed, but the whole phosphate layer is frequently streaky and has an adverse effect on the paint system subsequently applied.
Specifically for the treatment of composite metals of the type mentioned above, it is known to carry out treatment, first by dipping and then by spraying, using phosphating solutions containing 0.5 to 1.5 g/liter of zinc, 5 to 30 g/liter of phosphate ions, 0.6 to 3 g/liter of manganese ions and customary accelerators (European Laid-Open Specification 60,716). The dipping treatment should in this case be at least 15 seconds, advantageously 0.5 to 2 minutes, and the spraying treatment should be at least 2 seconds, advantageously 5 to 45 seconds. Even using this process, composite metals containing cavities cannot be phosphated in a satisfactory manner.
The object of the invention is to provide a process, for phosphating composite metals containing steel and zinc surfaces and having cavities, which does not have the disadvantages of the processes mentioned above, but results in a satisfactory, continuous phosphate layer--without additional outlay on the process.